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Re^2: Never, never, never

by MarkusLaker (Beadle)
on Apr 24, 2005 at 11:24 UTC ( #450918=note: print w/ replies, xml ) Need Help??


in reply to Re: Never, never, never
in thread Never, never, never

Here's a demonstration of why indirect object notation is dangerous:

#!/usr/bin/perl use warnings; use strict; package FH; sub new { bless { }; } sub Print { print "Running FH::Print\n"; } package main; sub Prunt { print "Running main::Prunt\n"; } my $fh = FH->new; print '$fh is an ', ref $fh, "\n"; Print $fh; $fh->Print;
Try running this program. Now rename Prunt as Print and watch Perl do something unexpected.

The explanation is that print $fh makes Perl look for Print in the current package before looking in $fh's package, whereas $fh->Print looks only in $fh's class and any base classes.

On another subject completely, this demo program violates another 'always, always, always' rule that people often mention: always pass the class name as the first argument of a constructor and bless the new object into the package supplied to you rather than the current package. I think that's unnecessary. Any derived package can call your constructor and then rebless the new object into itself.

IMHO, one of several problems that make OOP unnecessarily difficult in Perl 5 is that a constructor can't easily and efficiently tell whether it was invoked as pkg->new or pkg::new (in other words, whether the package name was passed as the first arguemnt or not, and therefore where in @_ the 'real' arguments begin). Most people use the former, but the latter is more efficient because it passes one argument fewer. Perl 6 fixes this problem, along with so many others.

Markus


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